\$\begingroup\$zanman, I've rolled back your edit that includes the tweet from Jeremy Crawford. If it answers the question you are encouraged to answer your own question but please post it in the appropriate answer section :) If you do post it as an answer please note also that you should quote the relevant text in your answer, not just leave the link.\$\endgroup\$
– Purple MonkeyMay 16 '18 at 4:32

2 Answers
2

So, a few minutes after I posted this question, I found some information from Jeremy Crawford which explained how it didn't work. One of his answers here says that it says "a Medium creature", which does not include creatures on top of other creatures:

Twitter user: Just so you know, the #DND hive mind has already decided that a medium centaur can ride a centaur riding a centaur ... (repeat) ... riding a centaur. The puzzle is solved. It's centaurs all the way down.

In response, someone else asked him to clarify whether centaurs could ride other centaurs; another tweet by Crawford explained that mounts must have suitable size and anatomy to ride it:

Twitter user: Doesn't that still mean that a centaur can ride another centaur?

Crawford: The rules on riding a mount specify that a mount must have a suitable size and anatomy for you to ride it (PH, 198). If the game makes an exception to part of that rule—about the creature's size, for instance—the other part of the rule still stands.

While I could personally see the argument that only one Medium creature would be "riding" the centaur, the second makes the centaur deathstack invalid.

No, There are no such limitations.

That said, this is Unearthed Arcana material and requires even more finishing than regular 5e material to be ready for play. Simple solutions include making Centaurs Large (I mean, horses are Large and regular Centaurs are large so...), generally prohibiting Towers of Rage by ruling that mounted creatures cannot themselves be mounted, or just ruling that centaurs can't mount stuff in general or always count as mounted and you can't mount more than one creature or similar.

But no, in the base game with the inclusion of this Unearthed Arcana there's nothing stopping this from working.

\$\begingroup\$@Medix2 That's what I was thinking as well - the carrying capacity rules prevent stacking centaurs given that a typical horse weighs > 1,000 lbs.\$\endgroup\$
– RobertFMay 16 '18 at 15:19